The objective of the proposed research is to increase our understanding of the regulation of gene expression in human cells through the study of spontaneous and induced mutants of diploid fibroblasts and lymphoblasts in culture. We will seek mutants that have quantitatively or qualitatively altered levels of urea cycle enzyme expression. They will be analyzed biochemically and genetically to determine the enzyme activities affected, the gene products altered, and the chromosomal location and dominance relationships of the new mutations. Mutants resistant to the arginine analogue canavanine have already been isolated from human lymphoblasts. They contain high levels of argininosuccinate synthetase activity and this activity is refractory to the repression caused by arginine in normal lymphoblasts. The physical, kinetic and immunochemical properties of argininosuccinate synthetase, the rates of enzyme synthesis and degradation, and the translation and transcription of mRNA specific for argininosuccinate synthetase in normal and canavanine-resistant cells will be studied. Analysis of human cells with altered control mechanisms should enhance our understanding of growth, both normal as in differentiation and abnormal as in cancer.